Many organic compounds exist in the form of enantiomers, pairs of stereoisomers that are mirror-images of each other. Such compounds are said to exhibit the property of being chiral, which can be measured by optical rotation of plane polarized light. Chirality is of vital importance in the manufacture of pharmaceuticals, pesticides, and other biochemicals. Indeed, with many compounds, a +(or right-handed) enantiomer may have one biological activity, while the -(or left-handed) enantiomer has a completely different activity. Racemic mixtures of enantiomers, comprising half of each type, are relatively easy to prepare by existing methods, but are of little use in preparing pure enantiomeric intermediates and final products. Synthesis of pure enantiomer preparations requires use of chiral substrates, either as starting materials or as intermediates.
One useful step in industrially-relevant synthesis of organic chemicals is the oxidation of alkenes, otherwise known as olefins, to produce the corresponding epoxide. Accordingly, the search for methods of asymmetric synthesis of epoxides from olefins has been the subject of many chemical and biological studies. Olefins are abundantly available as natural products and as compounds produced by the chemical industry. Asymmetric epoxides, whether produced from olefins or by other reactions, have many advantages as electrophilic intermediates for stereochemical chemical syntheses involving reactions with nucleophiles.
1. Chemical Synthesis
Chemical epoxidation reactions using synthetic catalysts have been used to produce chiral epoxides. (Bolm, C. Angew. Chem. Int. Ed. Engl. 1991, 30, 403). These reactions require catalysts, produce generally poor yields, and require substituted or large olefins to be effective.
The development of chemical methods for the production of chiral epoxides started with the work of Henbest et al. (1967 J. Chem. Soc., Chem. Commun. 1085-1086) who developed homochiral (enantiomerically pure) percamphoric acid as a method for asymmetric epoxide synthesis. The enantiomeric purity of epoxide product was poor.
Sharpless and coworkers (Katsuki et al. 1980. J. Am. Chem. Soc. 102, 5974-5976; and Gao et al. 1987 J. Am. Chem. Soc. 109, 5765-5780) developed a catalytic chemical oxidation for the synthesis of asymmetric epoxides, using a metal (titanium) and allelic alcohols--those with an -OH group on the carbon adjacent to the alkene double-bond.
Groves and Myers (1983 J. Am. Chem. Soc. 105, 5791-5796) modified iron porphyrins to include optically active functionalities at the meso positions and investigated asymmetric epoxidations of prochiral olefins with these catalysts. Various substituted styrenes and aliphatic olefins were epoxidized with ee (enantiomeric excess, the difference between the concentration of one enantiomer and the other) values ranging between 0% for 1-methyl cyclohexene oxide and 51% for p-chlorostyrene oxide. Groves and Visk (1990 J. Org. Chem. 55, 3628-3634) used a chiral, vaulted binaphthyl porphyrin derivative to obtain enantiomeric excesses in the range of 20-72% for catalytic asymmetric epoxidations of certain olefins.
Tani et al. (1979 Tetrahedron Lett. 32, 3017-3020) reacted prochiral squalene, a 25 carbon linear olefin with multiple alkene double bonds, with ter-butyl peroxide and molybdenum (VI) catalysts in the presence of optically active diols to produce chiral epoxides.
Curci et al. (1984 J. Chem. Sot., Chem. Commun., 155-156.) reported the asymmetric epoxidation of unfunctionalized alkenes by dioxirane intermediates generated from potassium peroxomonosulphate and chiral ketones. Enantiomeric excesses were low, in the range 9-12%.
Sinigalia et al. (1987 Organometallics, 6, 728-734) describe the asymmetric epoxidation of simple olefins catalyzed by chiral diphosphine-modified platinum(II) complexes. They carried out the epoxidation of 1-octene and propene with dilute hydrogen peroxide and formed epoxide product with ee% as high as 41%.
O'Malley and Kodadek (1989 J. Am. Chem. Soc. 111, 9116-9117) report the synthesis and characterization of a "chiral wall" consisting of a tetra naphthalene derivation of porphyrin which can function as a catalyst in sodium hypochlorite epoxidation reactions. The reaction gives only partial asymmetric synthesis and a turnover number around 13-14 events per min.
Naruta et al. (1991J. Am. Chem. Soc. 113, 6865-6879) modeled cytochrome P-450 epoxidations by preparing a chiral C.sub.2 symmetric "twin coronet" porphyrin having chiral biaryl auxiliaries linked by ethereal bonds on both faces. They obtained enantiomeric excesses as high as 72% with some styrene derivatives.
Zhang et al. (1990 J. Am. Chem. Soc. 112, 2801-2803) used (salen)manganese complexes for enantioselective epoxidation of unfunctionalized olefins. Their first report described the synthesis of manganese complexes of chiral Schiff bases that catalyze epoxidation of alkyl- and aryl-substituted olefins. In a subsequent publication, Zhang et al. (1991 J. Org. Chem. 56, 2296-2298) described a method for the asymmetric epoxidation of cis-beta-methylstyrene with yields as high as 86% using NaOCl (sodium hypochlorite) and a chiral Mn(III) salen complex. They reported approximately 35 turnovers before the catalyst became inactivated. A revised procedure using catalysts derived from 1,2-diaminocyclohexane was reported by Jacobsen et al. (1991J. Am. Chem. Soc. 113, 7063-7064).
Irie et al. (1990. Tetrahedron Lett. 31, 7345-7348) also reported catalytic asymmetric epoxidation of unfunctionalized olefins using manganese-salen complexes. Their highest enantioselectivity of 50% was realized with phenyl propene. Other salen catalysts have been developed by Irie and coworkers (1991 Tetrahedron Lett. 32. 1055-1058; 1991 Synlett 2, 265-266).
Konishi et al. (1992J. Am. Chem. Soc. 114, 1313-1317) report on the asymmetric epoxidation of prochiral olefins such as styrene derivatives and vinyl naphthalene by iodosylbenzene. Oxidation was achieved by using the manganese complexes of the antipodes of p-xylylene-strapped porphyrin as catalysts in the presence of imidazole. The optically active epoxides were obtained in 42-58% ee.
Another chiral porphyrin catalyst has been reported by Halterman et al. (1991 J. Org. Chem. 56, 5253-5254). They prepared a chiral tetraphenylporphyrin exhibiting D.sub.4 symmetry. A manganese chloride complex of this porphyrin catalyzed alkene epoxidations and gave enantioselectivities in the range of 41 to 76%.
Recently developed synthetic catalysts allow for the epoxidation of some conjugated olefins (those with alternating single and double bonds) but olefins bearing only aliphatic substituents are poor substrates for these catalysts and other synthetic catalysts as well. (Colletti, S. L. and Halterman, R. L. Tetrahedron Lett. 1992, 33, 1005; Sinigalia et al. Organometallics 1987, 6, 728).
In sum, chemical synthetic methods are not highly enantioselective. The turnover rates are low, typically in the range of 1 to 20 turnovers per minute. The chemical catalysts are rapidly inactivated in the reactions, and high catalyst to substrate ratios are necessary.
2. Enzyme catalysis
Biological catalysis has been employed for epoxidation, employing purified enzymes or whole cells. (Ortiz de Montellano et al. J. Am. Chem Soc. 1991, 113, 3195; Boyd et al. J. Chem. Soc. Perkin Tram. 1, 1982, 2767; Schurig, V., Wismba, D. Angew. Chem. Int. Ed. Engl. 1984, 23, 796; Takahasi et al., Tetrahedron Lett. 1989, 30, 1583; Ohta, H., Tetsukawa, H. J. Chem. Soc. Chem. Commun. 1978, 849). Enzymatic epoxidation methods remain impractical in comparison with synthetic organic chemistry approaches.
a. Cytochrome P-450
The cytochrome P-450 monooxygenase family has been used for epoxidation catalysis both with purified enzymes isolated from mammalian, microbial and plant sources and with partially purified liver microsomal preparations. The stereochemical course and the enantioselectivity of the P-450 reactions have been examined in some instances.
Wislocki and Lu (1982 Proc. Natl. Acad. Sci. USA 79, 6802-6806) studied the epoxidation of 8-methylbenz[a]anthracene and showed that cytochromes P-450 and P-448 catalyze epoxidation at different faces of the 8,9-double bond. In 1982, Gelb et al. (1982 Biochem. Biophys. Res. Commun. In 1983 Ortiz de Montellano et al. (1983J. Biol. Chem. 258, 4208-4213) examined the stereochemistry of the epoxidation of 1-octene by cytochrome P-450. This reaction concurrently produced 1,2-oxidooctane and N-alkylation of the heme prosthetic group by the activated epoxide or the olefinic intermediate. Stereochemical analysis showed that the S (or -) enantiomer of the trans epoxide is formed in slight excess over the R (or +) enantiomer.
Yang and Chiu (1985 Arch. Biochem. Biophys. 240, 546-552) and Yang (1988 Biochem. Pharmacol. 37, 61-70) showed that cytochrome P-450 preparations have varying degrees of stereoselectivity in catalyzing epoxidation reactions at various double bond positions in a variety of polycyclic aromatic hydrocarbons.
Wistuba et al. (1989 Chirality 1, 127-136) detected partial enantioselectivity in the in vitro conversion of simple prochiral and chiral aliphatic alkenes into oxiranes by liver microsomes. The enantiomeric excess of the epoxides extended from 0% for trimethyloxirane to 50% for ethyloxirane.
The P-450 enzymes are not appropriate for large scale epoxide synthesis. They require a continuous supply of NADH plus an accessory enzyme in order to transfer reducing equivalents from NADH to the P-450 enzyme, and they are not highly enantioselective.
b. Other Enzymatic Epoxidations
Various oxidative enzymes and heine proteins have been tested for activity in catalyzing epoxidation reactions. Hemoglobin and myoglobin have limited activity in catalyzing epoxidations. Methemoglobin and metmyoglobin catalyze the hydrogen peroxide depended oxidation of styrene to styrene oxide and benzaldehyde. Equal amounts of the R and S enantiomers are formed (Capdevila et al., 1990 J. Biol. Chem. 265, 10865-10871).
Horseradish peroxidase (Ortiz de Montellano and Grab, 1987 Biochemistry 26, 5310-5314) is capable of catalyzing the cooxidation of styrene when supplemented with hydrogen peroxide and an oxidizable phenol. The key features of the reaction involve first the oxidation of the phenol to a free radical which in turn reacts with molecular oxygen to generate the peroxy radical. The peroxy radical then reacts with styrene to form the epoxide.
The stereoselectivity of the epoxidation of 7,8-dihydroxy-7,8-dihydrobenzo[a]pyrene by prostaglandin H synthase has been examined. The synthase showed little selectivity (Panthananickal et al., 1983 J. Biol. Chem. 258, 4411-4418).
Myeloperoxidase is capable of catalyzing epoxidation reactions. It generates primarily anti-diolepoxides from trans-7,8-dihydroxy-7,8-dihydrobenzo[a]pyrene in the presence of hydrogen peroxide. The anti/syn ratio of the anti-diolepoxides is greater than 5, suggesting that the epoxidation proceeds via peroxyl radical or a ferryl oxygen transfer-mediated reaction (Mallet et al., 1991 Carcinogenesis 12, 521-524).
The availability of these enzymes is limited. Moreover, most catalyze cooxidation reactions in which a cosubstrate is oxidized to form a peroxy radical intermediate which then chemically reacts with the olefin to form the epoxide product. Most enantioselectivity is lost in the cooxidation reactions. In short, these reactions are not highly enantioselective.
c. Cell cultures
The conversion of alkenes to epoxides by whole cell cultures of microorganisms has been reported. Ohta and Tetsukawa (1978 J. C. S. Chem. Comm., pp 849-850) studied the epoxidation of long-chain terminal olefins by whole cell cultures of Corynebacterium equi. They demonstrated the conversion of hexadec-1-ene (n-C.sub.14 H.sub.29 CH.dbd.CH.sub.2) to its corresponding R-(+) epoxide. Also, May and Schwartz (1974 J. Amer. Chem. Soc. 96:4031-4032) have shown that cell cultures of Pseudomonas oleovorans convert 1,7-octadiene to (R)-(+)-7,8-epoxy-1-octene. The P. Oleovorans cells contain a P-450 monooxygenase.
Stirling and Dalton (1979 FEMS Microbiology Letters 5:315-318) have shown that whole cell cultures of Methylococcus capsulatus (Bath) are capable of converting ethylene, propylene, 1-butene, and cis- and trans-2-butene to epoxides when supplied with formaldehyde as a cosubstrate.
Hou et al. (1979 App. and Envir. Microbiol. 38:127-134) isolated a number of methane-utilizing microbes and showed that resting cell-suspensions of 3 organisms (Methylosinus trichosporium, Methylococcus capsulatus and Methylobacterium organophilum) were capable of oxidizing C.sub.2 to C.sub.4 n-alkenes to their corresponding 1,2-epoxides.
De Smet et al. (1981 App. and Envir. Microbiol, 42:811-816) have shown that under optimum conditions, resting and growing cultures of Pseudomonas oleovarans convert 1-octene to 1,2-epoxyoctane.
Furuhashi et al. (1981 E. J. Appl. Microbiol. Biotechnol. 12:39-45) have shown that a gaseous hydrocarbon-assimilating microorganism, Nocardia corallina, grew on 1-alkenes (C.sub.3, C.sub.4 and C.sub.13 to C.sub.18) and produced corresponding 1,2-epoxyalkanes.
Fu et al. (1991 J. Am. Chem. Soc. 113,5878-80) reported that P. oleovarans cultures were unable to oxidize internal olefins and disubstituted terminal olefins.
d. Chloroperoxidase
Chloroperoxidase (CPO) is known to catalyze some olefin epoxidation reactions under certain conditions. Morris, D. R.; Hager, L. P. J. Biol. Chem. 1966, 241, 3582; Blanke, S. R.; Yi, S.; Hager, L. P. Biotechnology Lett. 1989, 11, 769. CPO catalyzes the classical one electron oxidations typical of plant peroxidases and possesses a potent catalase activity (Thomas, J. A.; Morris, D. R.; Hager, L. P. J. Biol. Chem. 1970, 245, 3129 and Ortiz de Montellano, P. R.; Choe, Y. S.; DePhillis, G.; Catalano, C. E. J. Biol. Chem. 1987, 262, 11641). CPO is similar to the P-450 cytochromes in that it catalyzes epoxidation and N-demethylation reactions (Kedderis, G. L.; Hallenberg, P. F. Arch. Bioch. Biophys. 1984, 233, 315). However, CPO utilizes H.sub.2 O.sub.2 whereas the P-450 enzymes utilize molecular oxygen and require a regenerable reducing reagent, usually NADH.
McCarthy and White (1983 J. Biol. Chem. 258, 9153-9158) used chloroperoxidase in the enzymatic oxidation of cyclohexene to its epoxide in the presence of hydrogen peroxide. Subsequently, Geigert et al. (1986 24 Biochem. Biophys. Res. Commun. 136, 778-782) used chloroperoxidase to catalyze epoxidation of propylene, allyl chloride, 1,3-butadiene, cyclopentene and styrene. These investigators found that under their reaction conditions, the enzyme was generally rapidly inactivated.
Ortiz de Montellano et al. (1987 J. Biol Chem. 262, 11641-11646) used chloroperoxidase and hydrogen peroxide to oxidize styrene to styrene oxide and phenylacetaldehyde. Using trans-[1-.sup.2 H]styrene they showed that the tram epoxide isomer was the product of the oxidation.
Elfarra et al. (1991 Arch. Bioch. Biophy. 286, 244-251) has demonstrated the NADPH-dependent oxidation of 1,3-butadiene by mouse liver microsomes and the hydrogen peroxide-dependent oxidation of 1,3 butadiene by chloroperoxidase. Both oxidations yielded butadiene monoxide and crotonaldehyde.
Asymmetric oxidation of sulfides catalyzed by CPO has been discovered recently. (Collonna et al., Tetrahedron: Asymmetry. 1992, 3, 95). However, CPO catalyzes a variety of peroxidative halogenation reactions that are not enantioselective. (Hager et al., J. Biol. Chem. 1966, 241, 1769). CPO-catalyzed halogenation of alkenes has been shown to result in a racemic mixture of enantiomers.
For example, when chloroperoxidase is supplied with chloride ion, hydrogen peroxide and cis or trans prophenylphosphonic acid, there is no stereoselective synthesis of the corresponding halohydrins (Kollonitsch et al., 1970 J. Am. Chem. Soc. 92, 444489-90). Likewise, chloroperoxidase produces racemic bromohydrins when propylene and styrene serve as halogen acceptors (Neidleman, S. L., and J. Geigert 1986 Biohalogenation: Principles, Basic Roles and Applications. John Wiley and Sons, publisher, page 109).
Also, Ramakrishnan et al. (1983 Biochemistry 22, 3271-3277) showed nearly complete lack of stereoselectivity in the reaction of 2,methyl-4-propylcyclopentane-1,3-dione to the corresponding chlorinated compound. CPO is neither regiospecific or stereospecific in these cases.
In short, despite substantial research efforts worldwide, no reliable method of enantioselective chloroperoxidase-catalysed epoxidation of alkenes has previously been developed. Such a method, and the enantiomerically pure epoxides it produces, is very desirable for many purposes, including chemical synthesis of chiral products.